Field of Invention
This invention relates to transducers that convert mechanical vibrations into electrical signals and, in particular, to a single-coil, hum-cancelling pickup for stringed musical instruments.
Discussion of Prior Art
A pickup is a transducer that senses mechanical vibrations produced by a musical instrument, often a stringed instrument such as an electric guitar, and converts the vibrations into an electrical signal that is amplified to produce musical sounds through a loudspeaker. Most electrical instruments use a magnetic pickup, while acoustic instruments often use a piezoelectric pickup.
A magnetic pickup generally consists of a magnet, or a set of magnetic pole pieces, wrapped with a single coil of several thousand turns of fine insulated wire. The pickup is mounted near the strings of an instrument, generally on the body of the instrument, but can also be mounted to the bridge, neck or pickguard such that the strings (generally made of a magnetic material, such as steel) are magnetized by the magnetic field of the pickup. When the strings vibrate, as when, for example, the instrument is being played, the vibrations disturb the magnetic field, change the magnetic flux and induce a signal through the pickup coil.
When playing an instrument with a single-coil magnetic pickup, a hum can be heard through the pickup during quiet sections of the music. The pickup acts as a directional antenna and is prone to picking up, along with the musical signal from the string vibrations, interference from ambient magnetic fields, such as those caused at the frequency of mains electricity (“mains hum”). Common sources of mains hum include, for example, amplifiers, processors, mixers, motors, power lines, or other electrical equipment located in proximity to the instrument.
A humbucking pickup, or humbucker, is a type of magnetic pickup designed to cancel, or “buck,” mains hum. A humbucker generally involves two coils instead of one. The two coils are wound in opposite directions of one another and are most often connected in series. Each coil is coupled with a separate magnetic field (magnet or magnetic pieces), the magnetic fields being opposite in polarity. Generally, one of the coils has the north pole of its magnetic field oriented toward the strings (“up”), while the other coil has the south pole of its magnetic field oriented up.
Interference from mains electricity reaches the two coils as common-mode noise, inducing an approximately equal signal in each coil. Since the coils are wound in reverse of one another, the induced signals from mains hum cancel each other out. Meanwhile, the magnetic fields of the coils being out of phase (opposite polarity) in conjunction with the coil windings being out of phase (reverse windings) result in the string signals induced in each coil being in phase with one another. Thus, if the two coils are connected in series, the total string signal is effectively doubled.
Although dual-coil humbuckers effectively reduce mains hum, precise hum cancellation is not possible because the two coils, being paired with separate magnetic fields, are positioned at different physical locations along the strings. Therefore, each coil picks up slightly different disturbances in the magnetic field with which it is coupled, both from the vibrating strings and from mains electricity, such that, despite the phase cancellation, a slight amount of residual “noise” remains in the musical signal.